An electrocardiograph used for detecting an electrocardiographic complex of a person, such as a driver, in a vehicle has been known. Such an electrocardiograph detects the electrocardiographic complex of the person (hereinafter referred to as the subject) by measuring an electric potential difference generated between induction electrodes disposed in a steering wheel.
To properly detect the electrocardiographic complex of the subject by using such an electrocardiograph, the electrocardiograph needs to realize that the body of the subject is in contact with each of the induction electrodes. As a method of realizing that the body is in contact with each of the induction electrodes, it is considered to simulate an electrocardiographic complex and to apply a signal (hereinafter referred to as the simulation noise signal) whose signal level is higher than a predetermined level to one of the induction electrodes. Such a method is described in JP-B2-3906703, for example.
FIG. 8A shows an assumed electrocardiograph 100 for a vehicle, which employs the method described in JP-B2-3906703. The assumed electrocardiograph 100 includes a first induction electrode ER1, a second induction electrode EL1, a first indifferent electrode IR1, and a second indifferent electrode IL1. The first induction electrode ER1 and the first indifferent electrode IR1 are paired with each other, and are located in an area of a steering wheel SW1 held with the right hand of the subject. Likewise, the second induction electrode EL1 and the second indifferent electrode IL1 are paired with each other, and are located in an area of a steering wheel SW1 held with the left hand of the subject. The assumed electrocardiograph 100 applies a simulation noise signal Sn to the second induction electrode ER1.
As shown in FIG. 8B, the assumed electrocardiograph 100 further includes a differential amplifying circuit 101, a filter 102 and an amplifier 103. The differential amplifying circuit 101 outputs an electric potential difference between a signal outputted from the first induction electrode ER1 and a signal outputted from the second induction electrode EU. The amplifier 103 amplifies the output of the differential amplifying circuit 101 after passing through the filter 102.
In the assumed electrocardiograph 100, in a state where the subject does not hold the steering wheel SW1, an impedance between the first induction electrode ER1 and the second induction electrode EU is infinity. Therefore, an output from the amplifier 103 has a value corresponding to the signal level of the simulation noise signal Sn.
On the other hand, in a state where the subject holds the steering wheel SW1 with both hands, the impedance between the first induction electrode ER1 and the second induction electrode EU becomes small. Therefore, the output from the amplifier 103 has a value lower than the signal level of the simulation noise signal Sn. Accordingly, the assumed electrocardiograph 100 can determine whether the steering wheel SW is held with both the hands or not.
However, in a state where the subject holds the steering wheel SW1 with only the left hand, the output from the amplifier 103 has a value lower than the signal level of the simulation noise signal Sn. Therefore, the assumed electrocardiograph 100 will erroneously determine that the steering wheel WS is held with both the hands.
Namely, it is difficult to properly detect that the body of the subject is in contact with both of the first induction electrode ER1 and the second induction electrode EU by the assumed electrocardiograph 100.